interesting. That one has the same kind of hastily machined bolt knob so the bolt on this one was probably just turned down. I thought it might have been machined to the cylinder shape from the more round shape you see on most of them.
Yeah the "barrel" shaped bolt knob is a late war feature that several of the Arsenals used (Nagoya and Jinsen for example) while others (Like Kokura and Toyo Kogyo) used the rounder/plum shape through the end of the war. Both style knobs appeared on the Naval specials.